1. Field of the Invention
The present invention relates to an insulated gate bipolar transistor (IGBT) for use as, e.g., a switching element in the field of power electronics.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
An insulated gate bipolar transistor (IGBT) is noted for high efficiency and fast switching, and used as a switching element in the field of power electronics.
The insulated gate bipolar transistor (IGBT) is classified into a PT (punch-through) structure and a NPT (non punch-through) structure. The punch-through structure includes a collector layer of a semiconductor substrate, and further includes a buffer layer and a drift layer epitaxially grown on the collector layer in this order. When a drift layer is required to have a high voltage resistance characteristic, it is required to have a thickness corresponding to the required high voltage resistance. This in turn increases the production cost due to the thicker epitaxial growth of the drift layer.
On the other hand, in the NPT structure, the drift layer is formed by grinding an FZ (Float Zoning) wafer. Therefore, even in case where a high voltage resistance characteristic is required, an increase in production cost can be suppressed.
In the case of the NPT structure, however, the collector layer is formed by injecting a low dose amount of P+ type impurities in the drift layer, and therefore the amount of holes to be injected into the drift layer from the collector layer becomes lower by several digits as compared with the PT structure. In this case, the discharge amount of the holes from the emitter electrode cannot be neglected.
In order to solve the aforementioned drawbacks, conventionally, several attempts have achieved to control the discharge of holes. For example, Japanese Patent Application Publication H8-167716 discloses the following structure. In the first region between adjacent trenches, the emitter region and the emitter electrode are connected to form a main cell in this region. On the other hand, in the second region between adjacent trenches which is different from the first region, an interlayer insulation film is formed between the emitter region and the emitter electrode to thereby form a dummy cell in this region. Holes are injected into the base layer from the collector side when the IGBT is in an “on” state. However, this structure decreases the area of the main cell, which makes it difficult to discharge holes to the emitter side via the main cell.
As discussed above, in the invention proposed by the aforementioned document, by positively decreasing the area ratio of the main cell, the discharge amount of holes to the emitter side can be controlled to keep the accumulation amount of holes, which enhances the conductivity modulation of the base layer. The aforementioned conventional technique, however, causes decreased current density due to the decreased area ratio of the main cell.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.